The long-term goal of the proposed research is to understand the role of the MPM-2 mitosis-associated phosphorylation site in regulation of microtubule associated protein (MAP) function. Hyperphosphorylation of tau has been implicated in inhibiting the binding of tau to microtubules and promoting the self-assembly of tau into paired helical filaments (PHF). PHF-tau is associated with degeneration of hippocampal and cortical neurons in Alzheimer's disease. The peptidyl cis/trans proline isomerase Pin l has been shown to recognize the MPM-2 phosphorylation site on a number of mitotic regulatory proteins, and recently tau. The working hypothesis for this proposal is that Pin 1 proline isomerase interacts with and functionally regulates MPM-2 epitope containing proteins in neuronal cells. The focus of the current work is to determine whether Pin 1 interacts with additional brain MAPs, and determine how Pin1 functionally regulates phosphorylated forms of tau and other brain MAPs. The distribution of Pin I in cultured neuronal cells, and normal and diseased brain tissue will be determined. Proteins that bind to Pin 1 will be identified and compared with MPM-2 reactive proteins present in neurons. One MPM-2 reactive protein is PHF-tau, and hyperphosphorylated forms of tau bind Pinl. The biochemical characterization of the tau MPM-2 epitope site will be completed and compared to the known pin1 binding site. Site directed mutation or selective phosphorylation of the MPM-2 epitope site will be utilized to examine the effect on binding of tau to microtubules, tau dimerization, and tau filament formation. Recombinant Pin1 isomerase will be prepared and incubated with MPM-2-reactive or mutant forms of tau lacking the MPM-2 phosphorylation site. The interaction of proline isomerase with selected phosphorylated tau isoforms indicates that conformational changes induced by the proline isomerase may play a role in either enhancing or inhibiting tau dimer or filament formation. These results will also define the extent that proline isomerases may interact with MAPs and other cytoskeletal proteins in neuronal cells. Proline isomerases that interact with the MPM-2 epitope in neuronal cells may provide unique target molecules for therapeutic intervention in neurodegenerative diseases.